The present invention relates to modems, and particularly to packet data radio modems, and the radio communication systems incorporating such modems. In further particularity, the present invention pertains to ensuring that the frequency at the receiving modem matches the carrier frequency of the transmitting modem.
Modems modulate digital data onto analog carrier signals for transmission over analog communication channels, and demodulate received signals to recover the digital data. The analog communication channel may be a conventional wired telephone system, or it may be a wireless radio link.
The modes of modulating the data onto the carrier signal in radio communication links may be different than the modes in a wire system. A radio communication link provides a number of complications for a modem using that link to transmit data.
One technique used by modems for use in radio communication systems is to package the data to be transmitted into discrete packets of data. The data packets are frequency modulated onto the carrier signal. The receiving modem recovers the data from the frequency modulated signal.
In a radio communication link such as is used with a packet data radio modem, the carrier signal may not always be present. The periodic lack of a carrier signal may cause the receiving modem to conclude that the communication link has been lost, or to erroneously alter its expectations of the received carrier frequency.
As is well known, the quality of a radio transmission link can vary considerably during a single communication session. The radio transmission link may be substantially less than ideal. When the radio link is poor, the receiving modem may misinterpret the data information contained in the received signal. The rate at which errors are made is called the bit error rate. The bit error rate is affected by such factors as thermal noise, oscillator phase noise, group delay distortion, interfering sources, and frequency errors. The present invention reduces errors due to frequency errors.
The frequency at which the receiving modem is operating must match the frequency of the transmitting modem. In the case of a radio modem, the mobile modem must match that of the fixed base station with which it is communicating. There are events that occur in radio data transmission that may require that the mobile unit recalibrate its operating frequency. For example, as it moves, the mobile unit may switch to a different operating cell, which has a different fixed base unit. The mobile unit should also retain it knowledge of the carrier signal frequency when it loses the radio signal for short periods of time.
Various automatic frequency control techniques for locking the local oscillator of the mobile modem to the same frequency as that of the base station have been proposed. Such techniques include a local oscillator in the mobile unit that is to produce a frequency that matches the frequency of the transmitting station. Circuitry is used to analyze the output of the demodulator and determine if it appears that the carrier signal has been correctly removed from the received signal. However, such techniques generally require extremely accurate (and expensive) components, and careful calibration.
Thus, a low cost, simple arrangement for an automatic frequency control technique for locking the local oscillator of the mobile modem to the same frequency as that of the base station is desired. In addition, a low cost, simple mechanism is desired for maintaining the last known frequency during brief periods when the carrier signal is not present.